Interest in the recovery of CO2 from various CO2 containing gas mixture has been fueled by multiple factors: the merchant CO2 market, enhanced oil recovery (EOR) and greenhouse gas emissions reduction. However, the majority of CO2 sources are from low pressure gas mixtures having a relatively low concentration of CO2. Such sources, for example, include the flue gas from a fossil fuel-fired power plant, an industrial furnace, a cement kiln, an oxy or air combustion facility, or the exhaust gas of an engine or lime kiln. Typically, the flue gas is obtained at near ambient pressure (<3 Bara). The concentration of CO2 in the flue gas ranges from approximately 5 to 30%, with a balance of mostly nitrogen. The flue gas flow rate may be considerable.
Conventionally, most commercial CO2 recovery plants use processes based on chemical absorption with a monoethanolamine (MEA) solvent. MEA was developed over 60 years ago for removing CO2 and H2S from natural gas streams. However, this process suffers from large equipment costs and high regeneration energy requirements. Recently, a CO2 CPU (compression and purification unit) process was proposed to capture the CO2 from the off gas of a H2 PSA (pressure swing adsorption) treating a syngas coming from a SMR (steam methane reforming) plant. The benefit of the process is that the waste gas from the CPU plant, which normally contains significant amounts of H2 at high pressure, can be recycled back to the H2 PSA for additional H2 production credit. But the CPU process which requires high compression and cold temperature operation is not economically interesting when the CO2 concentration in the feed is low, such as in the case of flue gas.
The typical cycle time of a standard TSA is considerably more than one hour. Considering an example of Front End Purification (FEP) unit upstream an Air Separation Unit (ASU), which is one of the most advanced designs for a TSA unit, the adsorption time is usually between 2 or 4 hours. Heating plus cooling times takes 1.5 hr to 3.5 hr, and the remaining time corresponding to secondary steps, like depressurization, repressurization and possibly idle time as a margin. For large air flow, the adsorbers are normally designed to be a radial flow type, providing a large passage section for the air (50 m2 for instance) with relative short bed (less than 1 meter thick).
When trying to reduce the cycle time to less than 1 hour, one faces several problems. In order to be efficient enough in the removal of the impurities present in atmospheric air (water, CO2, C2H4, C3H8, N2O), the size of the adsorbent, bead or pellet, has to be reduced accordingly to maintain the same ratio of saturation zone/mass transfer zone. Instead of the 2 mm standard diameter, it leads to beads having a diameter of less than 1 mm, in the range of 0.5 mm with a target of an adsorption time around 15 minutes. With such an adsorbent particle size, limiting the pressure drop leads to non-industrial geometry with large section for the gas and particularly very short bed, in the order of 10 centimeters. Gas distribution in such adsorbers added to the construction tolerances to be respected, makes these designs unrealistic.
In the past decade, solutions were proposed to both decrease pressure drop (and avoid fluidization of the adsorbent) and increase the mass transfer rate. Different types of structured adsorbent were described and proposed for shorten the cycle time of Adsorption unit (PSA or TSA). For a complete review of the subject, one can refer to the document “Structured adsorbents in gas separation processes” by F. Rezaei and P. Webley in Separation and Purification Technology.
In the case of CO2 capture, the flue gas is near atmospheric pressure and the energy cost of any pressure drop to be compensated by a compression means is tremendous and quickly makes the process uneconomic. For such a target, the adsorbent structure with the less pressure drop is to be selected. The preferred geometries will be the one with a direct passage for the gas. We call such adsorbent structures “parallel passages contactor”. They are of different types: monoliths (or honeycomb), laminates, fabrics, fibers bundle.
Monolith is entirely comprised of adsorbent with a binder to solidify the structure which is directly extruded. The gas passage consists of parallel channels going straight right from one side (inlet) to the other (outlet). The term of honeycomb structure is often used to describe this kind of contactor.
In case of laminate, the adsorbent is deposed or grown up on a substrate support, for instance a sheet of special paper or a metallic grid . . . which is packed or rolled; if necessary, spacers are used to maintain a channel for the gas to flow through the structure.
A relative similar structure can be obtained with adsorbent fabrics (or cloths) in stacking parallel sheets or making a spiral wound adsorber. The gas passage is parallel to the surface of the fabrics (and not across the fabrics as in some filter).
Less common is fibers bundle, which is somewhere similar in geometry to permeation unit. The gas flows outside and/or inside the fibers (if hollow fibers) which are made from or covered with adsorbent.
Not only these parallel passages contactors will decrease dramatically the pressure drop in comparison with conventional particles adsorbent bed but they can increase by several folds the mass transfer and the local heat transfer of the system. This is realized through the small thickness of the adsorbent layer. We call “effective thickness” the length to be penetrated by the gas to reach all the adsorbent sites. In case of an adsorbent supported on an inert sheet, the effective dimension is directly the thickness of the layer (case of laminate . . . ). When the gas flows both sides of a wall consisting in adsorbent material, the effective thickness is half the thickness of the wall. Adsorbent effective thickness is often reported to be in the range 50 to 200 microns for efficient mass transport while the gas channels are in the range 100 to 400 microns. In case of supported adsorbent, the support itself is around 50 to 100 microns.
The main drawback of these structures is the adsorbent loading (per adsorber volume). For a standard bead bed, with a dense loading, one can obtain a 65% loading. Using the newly developed binderless adsorbent, the effective loading (i.e. counting only the active material) is also about 65%. If we compare with a laminate structure (support 50/layer 100/channel 200 microns), we obtain a 44% loading or taking into account the necessary binder (80/20) an effective loading of about 35%. For a low pressure application, such a design will lead to too much pressure drop and the width of the channel should be increased to about 1 mm. The effective loading will decrease down to 13%. It means that most of the gain expected in shortening the TSA cycle will be lost due to a low volumetric adsorbent load.
A second problem occurs when increasing the width of the gas channel. The film (or bulk) resistance i.e. in the gas flow itself increases, thus limiting the mass transfer and the local heat transfer (from the fluid to the adsorbent surface).
The gain in transport properties consecutive to the reduction of the adsorbent effective thickness and consequently the pore diffusion resistance will be less than expected due to the fact that the bulk resistance (external film resistance) will be pre-eminent. This means that the mass transfer zone will lengthen, that more adsorbent will not be saturated leading to a decrease in productivity.
Another difficulty is the heat transfer during the regeneration. In conversional TSA processes, the heat is brought through the circulation of a hot purge gas. The amount of purge gas available varies from one process to another. For the FEP upstream an ASU unit, the regeneration flow rate (nitrogen off-gas of the ASU) is for instance in the range of 10 to 30% of the air flow rate. When decreasing the cycle time, the trend is to increase that flow (one of the reasons is that the size of the heater, of the external piping . . . is not decreasing when shortening the cycle and thus that the time constant for the heat propagation to the adsorber remain the same, rather negligible for a 4 hours cycle but very sensitive for a 20 minutes cycle).
In case where the adsorbed species is the “valuable one”, its concentration will be diluted in too much purge gas and the whole process becomes inefficient.
For that reason, several solutions have been proposed: each adsorber contains its own heater inside or just at the inlet to decrease heat capacity of the external equipments (piping, valve . . . ) to be heated; several heaters at different places in the adsorber; in situ electrical heating; micro waves . . . . Another solution proposed to increase heat transfer in and out of adsorbents by designing adsorbers as a heat exchanger type (including heat exchange tubes inside adsorbent bed or coating adsorbent onto surface of heat exchange tubes).
Use of a vacuum pump to help desorbing the adsorbed species without too much diluting them has been used for some particular applications (sub marine atmosphere maintenance . . . ).
All these solutions are expensive (investment, energy consumption) or difficult to extrapolate for large flow rates. As a direct consequence of these various disadvantages, CO2 capture by TSA process is not presently a well established process. The present invention is likely to change this point of view.